The production of carbon nanotubes with controlled atomic and electronic structure has led to devices with improved performance and functionality, and enabled a more detailed understanding of the physical properties of these one-dimensional nanomaterials. Despite the many benefits, it remains a considerable challenge to generate carbon nanotubes with controlled structure. Synthetic methods can achieve some degree of control over the distribution of nanotube chiralities and their electronic type. For instance, single-walled carbon nanotubes (SWNTs) are typically synthesized with a statistical two-to-one ratio of semiconducting to metallic species. However, this polydispersity has hindered the development of many nanotube-based technologies and further improvements are required to optimize device performance. To address this issue, a number of post-synthesis methods of separating carbon nanotubes according to their diameter, wrapping angle, and electronic type (metallic versus semiconducting) have been developed. For instance, a large number of polymers and biomolecules, such as PFO, single-stranded DNA, and flavin mononucleotide, adopt structure-dependent configurations around SWNTs, which can be exploited to enable isolation according to SWNT structure. The SWNT structural specificity of these molecules is generally attributed to their ability to self-associate and form sheath-like structures that conform with the atomic structure of a given SWNT species.
One of the leading methods for separating SWNTs is density gradient ultracentrifugation (DGU). Without limitation, this technique exploits differences in the buoyant density of SWNTs encapsulated by surfactants, which translate into differences in the position of the SWNTs once subjected to high centripetal forces in density gradients. Previous work has shown, for instance, that by simply changing the levels of anionic co-surfactants sodium cholate and sodium dodecyl sulfate present in a density gradient, it is possible to isolate SWNTs according to diameter and/or electronic type with purity levels for the latter exceeding 99%. Despite some experimental and theoretical study, the surfactant-SWNT interactions that enable DGU separations, particularly those by electronic type, are not well understood. This limited understanding is due in part to the difficulty in faithfully simulating a typical DGU experiment, which involves the complex interplay between SWNTs, different mixtures of competing surfactant species, counterions, water molecules, and density gradient media.
In the absence of detailed theoretical understanding, improving the fidelity and yield of DGU separations will remain an often laborious process. Accordingly, there remains an on-going effort in the art to develop a nanotube separation system to better understand and utilize the benefits available through DGU.